In a conventional electrical storage device using a plurality of electrical double layer capacitors arranged in series connection, terminal voltage of each capacitor is detected through the following process. Each pair of two resistors connected in series is connected between each of the connecting points of capacitors, i.e., each terminal, and ground. The voltage at the connecting point of the two resistors (divided voltage) is fed into a selector switch. A microcomputer sends the selector switch a capacitor selecting signal to control the selector switch. Receiving the signal, the selector switch sends the divided voltage of the selected capacitor to the A/D converter of the microcomputer. In this way, the microcomputer calculates the divided voltage to find voltage between the terminals of each capacitor.
Such type of voltage monitor is disclosed in Japanese Patent Unexamined Publication No. 2004-271356. The aforementioned voltage monitor employs a multiplexer as a selector switch so that the multiplexer selects a connecting combination of a plurality of resistors.
However, the input terminals of the multiplexer are disposed at narrow intervals. When such a multiplexer is mounted as the selector switch on a circuit board, a short-circuit may occur between adjacent input terminals because of migration of metals constituting electrodes. This can be a cause of an inaccurate detection of the divided voltage of each capacitor. To avoid this, such a short-circuit is required to be detected.
As a method of detecting a short-circuit between the input terminals, the following is possible; for example, connect the input terminals located on the both sides of the input terminal connected to the output of divided voltage to ground, instead of connecting all the adjacent input terminals of the capacitor to the output of divided voltage. If a short-circuit occurs between the adjacent input terminals, the short-circuit is easily detected because the output of divided voltage is fixed to ground. However, the above-mentioned connecting method is not efficient, since the input terminals are alternately connected to output and ground, so that effectively usable number of the input terminals is reduced to half. Alternatively, a larger selector switch is required to be use, however, employing the larger selector switch inconveniently increases the size of the voltage monitor.